The operation of the optical fiber backscatter generator (OFBSG) is difficult to describe without briefly describing an optical time-domain reflectometer (OTDR) first. An OTDR measures attenuation and locates breaks in a fiber optic cable by detecting optical power that is scattered backwards from forward traveling optical pulses. These short duration pulses are generated by a laser in the OTDR and launched into the cable under test. The backscatter power is caused by Rayleigh scattering or Fresnel reflections. Splice losses, microbends, and breaks can be measured by detecting this backscatter power as a function of "round-trip" time for the pulses to travel into the cable and scatter backwards. The detected power decays exponentially as a function of distance into the cable due to Rayleigh scattering. Looking to FIG. 1a an illustration of OTDR data is presented which shows the measured backscattered power at a wavelength of 1.55 microns from a 50 kilometer long single-mode optical fiber. The backscatter decays exponentially, but is displayed in a linear format in FIG. 1a by taking five times the Logarithm of the backscatter data. The data indicate an attenuation of 0.21 dB per kilometer and show a 0.23 dB splice loss at about 25 kilometers from the beginning of the fiber which is characterized by a discontinuity in the backscattered data. A large Fresnel reflection appears at the end of the fiber.
While, generally speaking, the OTDRs are fabricated to assure long-term reliable life, no practical device instrumentation or technique is known to be available on the market to test, monitor, or otherwise characterize the performance of these OTDRs. As a consequence, it is difficult to determine an optimum OTDR performance level or whether or not an OTDR is still performing satisfactorily.
Several techniques have been proposed in journal publications that utilize passive optical set ups. An apparent disadvantage of these techniques is that no single set up can test all of the different types of OTDRs without extensive modifications.
Thus, a continuing need exists in the state of the art for an optical fiber backscatter signal generator that generates optical signals used to characterize or calibrate an OTDR that is capable of monitoring different OTDRs and has a flexibility to accommodate that various types of OTDRs on the market and which lends itself for automatic testing thereof.